<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button id="btn">发送ajax请求</button>

    <script>
        btn.onclick = function () {
            var xhr = new XMLHttpRequest;

            xhr.open('get', 'http://localhost:3000/cache?t=' + Math.random());

            // 发送请求
            xhr.send();
            // 
            xhr.onreadystatechange = function () {
                if (xhr.readyState == 4 && xhr.status == 200) {
                    alert(xhr.responseText);
                }
            };

            // 当网络中断时，会触发onerror事件；
            xhr.onerror = function () {
                var a = '网络中断，无法发生ajax请求';
                alert(a);
                console.log(a);
            };
        }
    </script>
</body>

</html>
